Electric induction apparatus



-29, 1935. NQRTHRUP 1,989,376

ELECTRIC INDUCTION APPARATUS Filed Jan. 51, 1931 2 Sheets-Sheet lELECTRIC INDUCTION APPARATUS Filed Jan. 31, 1951 2 Sheets-Sheet 2 14MIMI/2361191? .17 4 kl I F. NQRTHRUP 1,989,376 I Patented Jan. 29, 1935UNITED 'STATES PATENT- OFFICE 1,989,376 ELECTRIC INDUCTION APPARATUSEdwin Fitch Northrup, Princeton, N. J., assignor to Ajax ElectrotherlnicCorporation, Ajax Park, N. .L, a corporation of New Jersey I ApplicationJanuary 31, 1931, Serial No. 512,536

23 Claims. (Cl. 219-43) My invention relates to induction electricfurnaces and especially to spacing and focus inducto'rs and methods ofusing the same.

A purpose of my invention is to heat a metallic 5 charge in the solidstate by progressing it through a spacing inductor, preferably carryingthe charge through a soaking pit in prolongation of the spacing inductorso that the temperature throughout the charge cross section may becomeuniform.

A further purpose is to produce a plurality of heated billets byprogressing a continuous charge through a spacing inductor heater andcutting from the heated end a suitable billet length.

A further purpose is to guide the progression of a charge through aninductor by a nonmagnetic metallic shell, slitted longitudinally andthin enough to avoid substantial inductive heating in the best form.

A further purpose is to employ a nonoxidizing atmosphere in a spacinginductor heater.

A further purpose is to arrange afocus in-' ductor heater with separatefocus inductor loops on different axes, one beside the other, with a'primary inductor included within one of the loops and that loopshielded so that workmen are 7 protected from contact with the primaryinductor by the low voltage-focus inductor loop surrounding it and alsoby the shield, while the other focus inductor loop is accessible'andsafe in use.

- A further purpose is to extend a primary inductor axially somewhatbeyond a spacing inductor to increase the concentration of induction inthe ends of the spacing inductor, thus allow ing for losses peculiar tothe ends.

A further purpose is to construct a single turn focus inductor of aplurality of water cooled tubes electrically united by lateral strips incontact with the tubes.

A further purpose is to winda primary inductor of hollow flattenededgewound tubing while preferably using a spacing inductor in inductiverelation with the primary inductor of hollow flattened flatwound tubing.i

A further purpose is to use alternatively guides of different kinds, whether of ring shape or in the form of skids or otherwise.

Further purposes appear-in the specification and in the claims.

My inventionrelates both to the methods involved and to apparatus forcarrying out the methods.

In the drawings I illustrate one only of the many constructions whichmay be used to embody my bread invention, choosing the form shown fromthe standpoint of satisfactory operation and convenient illustration ofthe principles involved Figure 1 is a diagrammatic central verticalsection of my preferred form. v

Figure 1a is a fragmentary enlarged view of a portion of Figure 1,showing a modification in the charge.

Figure 2 is a section of Figure 2-2.

Figure 2a is a fragmentary enlargement of Figure 2.

Figure 2b is a fragmentary end elevation corresponding to Figure 2a andshowing a variation of the charge supporting means.

Figure 3 is an end elevation of Figure 1 position 3-3.

The inductive heating of a solid metallic charge has ordinarily beenaccomplished by passing it through a primary inductor coil directlyinducing current in the charge, or through a niufile 1 on the line whosewalls are inductively heated. While both of these forms of heating arehighly advantageous, each has features rendering it unsuitable incertain installations.

For example, where direct inductive heating is used, the inductor coilis ordinarily supplied with current at a fairly high voltage, requiringgood insulation between the inductor and the charge and a considerablequantity of insulation. The insulation between the inductor coil and thecharge must be capable of withstanding the temperature to which it issubjected by conduction from the charge. Frequently, it breaks downthermally before it ruptures electrically. The designer is not free toleave as much space for insulation between the inductor coil' and thecharge as he might otherwise wish because close coupling is required forgood power factor.

in the The high voltage inductor coil, being very close to the charge,is also likely to be inadvertently touched by a workman, andconsequently special precautions in the way of shields and insulationare needed whereotherwise it might be desirable to have the parts leftmuch more accessible.

In a direct induction furnace, the power factor drops very sharply whenthe charge is removed from thefurnace, with,a corresponding loss-inefficiency where it is not possible to operate the furnace continuouslyon full load.

The mufiie type of furnace has the inherent difficulty that the heat isnot developed directly in the charge, and it is necessary to allow forconduction and radiation of heat to the charge.

33 my present invention I avoid many of the difficulties met in directinduction from a primary inductor and in inductive muiile heating, whileat the same time I retain many of the advantages of these other forms.My invention is to some extent a continuation of those disclosed in mypatents, No. 1,378,187 for Focus inductor furnace, granted May 17, 1921and No. 1,378,138 for ladle heating by high frequency currents, grantedMay v an explanation of the general principles involved.

As in these patents, I transfer energy-to a charge from a primaryinductor through a spacing inductor, or; in some cases, a focusinductor;

By a spacing inductor I mean to designate broadly an inductor ininductive relation to a primary inductor and also in inductive relationto a charge, which transmits energy over a relatively short distancefrom the primary inductorto the charge, without regard to whether or notany concentration of input takes place, either because the spacinginductor has fewer turns than the primary inductor and therefore carriesa larger current, or because the length of the spacing inductor ininductive relation to the charge is less than the length of the spacinginductor'in inductive relation to the primary inductor, or for any otherreason.

By a focus inductor, on the other hand, I mean to designate one of aclass of spacing inductors which focuses or concentrates energy, eitherbecause it has fewer turns than the primary inductor with which it iscoupled, or because it has a smaller length in inductive relation to thecharge than in inductive relation to the primary inductor, or for anyother reason.

Certain aspects of my invention are applicable broadly to inductionfurnaces, and even in the narrower aspects of my invention, I do notrestrict myself to the type of spacing inductor or of focus inductor inwhich the spacing or focus inductor loops have separate axes or to thetype in which the loops are concentric, except in those featuresapplicable only to the nonconcentric form.

In the drawings I show a source of alternating current 6 connected byleads 7 and 8 to a primary inductor 10 of hollow flattened edgewoundtubing, supplied with water through suitable connections at 11 and 12.The power factor is desirably corrected by capacitative reactance 9. Ininductive relation to the primary inductor 10 is located a spacinginductor 13 having loops l4 and 15. The spacing inductor 13 is made upof a plurality .of bands, each comprising a flattened flatwound tube,connected by bars 16 and 17 welded respectively at the top and bottom ofeach band, so that electrically the spacing inductor comprises only oneturn axially as well as annularly. The spacing inductor is cooled bywater flowing from a I plurality of inlet connections 18 to a pluralityof outlet connections 19.

, Since the primary inductor and the spacing inductor are stationarywith respect to one another, and since no heating takes place close tothe surface of contact between them, the separating insulation 20between the primary inductor and the spacing inductor loop 14 may beextremely thin, insuring very close coupling. I find that in practice afew sheets of mica are satisfactory.

The loop 15 of the spacing inductor surrounds the charge 21. The charge21 is guided'through the spacing inductor by the walls of a thin non- Imagnetic metallic shell 22, preferably of austenitic iron. The shell 22.will ordinarily be of a thickness less than the depth of penetrationfor the frequency chosen. The shell 22 conforms closely to the crosssection of the ,charge 21 and is slitted longitudinally at 23 to avoidexcessive inductive heating of the shell itself. Because of its'thin,

walls, nonmagnetic characterand longitudinal slit, the shell 22 is inno'sense a muilie, but is rather a heat resisting guide for the chargeand guard for the interior of the inductor coil.

Between the spacing inductor loop 15 and the shell 22 I'place suitableelectrical insulation 24.

vented by ears 25 extending from the ends of the shell and bent overthe-ends of the spacing inductor as shown in Figure 2a.

- The utility of the shell 22, while it is likely to begreatest in focusinductor furnaces because of the low voltage on the inductor close tothe charge, is by no means restricted to such furnaces, and will bepresent to some extent in induction furnaces generally.

As an alternative means of support and guidance of the charge I may userods or tubes as skids 22', as seen in Figure 2b. The skids 22 may beinsulated from the inductor coil or may be in direct contact with theinductor coil.

In the latter'case, the skids will be cooled by heat transfer to theinductor coil.

At-26 I show an inlet to the interior of the shell 22 suitable foradmitting a 'nonoxidizing gas to prevent oxidation of the shell andscaling of the charge. The charge 21 may be uncut as shown in Figure1,'or it may be already cut to length as shown at 21' in Figure la. Thecharge is directed into the shell 22 by a guide 27, and as soon .as thecharge enters the shell it is concurrently shell, preferably beingprotected in the meantime by nonoxidizing gas.

Particularly at the higher frequencies, but to some extent even atmoderate and low frequency,

inductive heating is largely concentrated in the outside of the charge.If the inductive heating be'very rapid,'the outside portions of thecharge cross section will be at higher temperature than the insideportions after heating. To permit uniform-distribution'of the heat, Iprovide a soaking pit 28, axially beyond the heating chamber in thedirection of charge travel and preferably formed of heat insulatingmaterial to prevent too rapid loss of heat. The length of the soakingpit will of course depend upon the rate of heating and the degree ofuniformity required.

At the end of the heating chamber or at the outlet end of the soakingpit where soakingis used, the charge may be put to any use required.Where a continuous charge, such as a long rod, is to be shaped, I mayvery desirably cut the heated charge to the size desired for the shapingmachine. As an illustrationI show a circular saw 29, on a movablesupport 30, for periodically cutting the charge. It will of course beunderstood that I may heat a succession'of relatively small separatecharges instead of a continuous charge, and that where separate chargesof proper size are used, no cutting subsequent to heating will beneeded. I show a small unit charge in Figure 1a.

I will preferably use as many turns as possible in the primary inductorand as few turns as possible in the spacing inductor. In Figure 1 Iillus trate a large number of primary inductor turns and only onespacing inductor turn. I make the individual spacing inductor tubesflatwound as seen at 31 sothat they will cover as wide a space aspossible, while I edgewindthe primary inductor in my preferred form so.that a maximum number of primary'turns may be in inductive relationWhere a high voltage is impressed on the primary inductor, as willnormally be the case, the spacing inductor voltage will nevertheless bequite low, so that workmen will not be endangered. In the structureshown, the sides of the primary inductor 10 are very largely covered bythe spacing inductor 13. This is distinctly advantageous from thestandpoint of safety.

in operation and can be handled freely and with safety. In case a chargesticks, a' workman may safely insert a metal rod into the furnacewithout danger to himself even in case of breakdown in the insulation.

Since cooling is more rapid at the ends of the heating chamber, Ipreferably somewhat increase the inductive input at the endsby extendingthe primary inductor 10 axially at 33 somewhat beyond the spacinginductor 13. In this way end tubes 34 of the spacing inductor, in spiteof the strips 16 and 1'7, carry slightly heavier current than othertubes, inducing more current in the charge.

Actual experiments made by me show a remarkably good power factorregulation. From full load to no load the change in power factor hasbeen only about 7%.

It is diflicult to explain this very lowpower factor change when thecharge is removed, but I consider that, among other factors, it is dueto the fact that the spacing'inductor provides a secondary circuit forthe primary inductor which is closed at all times without regard to thepresence ,of the charge in the spacing inductor or its absencetherefrom. The spacing inductor/voltage is low, and this permits veryclose coupling between the spacing inductor and the charge. The couplingbetween the primary inductor and the spacing inductor is also extremelygood. 1' There are doubtless other influences. Of those mentioned, thefact that the spacing inductor always extends forth as a load on theprimary inductor is probably the most important.

With a magnetic charge and high frequency current the current is inducedin an extreme outer layer first with the result that viewed from the endan annulus of the material is readily heated to a cherry red. Apparentlyas soon as this outer layer loses its magnetism the current is inducedwithin a corresponding outer layer of charge which still remainsmagnetic so that an adjoining annulus as viewed from the end is raisedintemperature to correspond with a cherryred color. With continuedapplication of a high frequency current the cherry red zone increasesprogressively from a mere ring at the extreme outer surface to a widering ultimately closing at the center. With lower frequencies of coursethe depth of ring initially heated would be increased and theprogressionof the color inwardly would be much less noticeable, withquite low frequencies heating much more nearly uniformly all the waythrough.

It would appear that certainly with the higher frequencies and to alesser extent as the frequencies are reduced the path of the inductionlies within the outer shell of the magnetic material and continues thereuntil this portion of the charge loses its magnetism when the current isinduced primarily not in this now non-magnetic rim but .in the outermostpart of that portion of the charge which still remains magnetic, be-

ing progressively maintained in the' magnetic material as long as any ofthe material is magnetic.

Whatever be the exact fact, whether the theory suggested above be.correct or not, the fact remains that the temperature is not uniformthroughout the entire'thickness of a cylindrical charge-at the time thatit is first raised in its entirety to a cherry red; and this givesopportunity for considerable advantage in uniformity of temperature fromsoaking the charge in a non-oxidizing atmosphere protected against heatleakage but free from any considerable current induction or at least nothaving more current I induction than will equalize the cooling from thesurface. My invention contemplates the possibility of using someinduction for this purpose if desired;

I will preferably employ moderate or high frequency current, desirablyof the order of several thousand cycles, in the furnace shown. However,many of the aspects of my invention are equally applicable to lowfrequency, and may be so employed. 4

In view of my invention and disclosure variations and modifications tomeet individual whim 'or particular need will doubtless become evidentto others skilled in the art, to obtain part or all of the benefits ofmy invention without copying thestructure shown, and I, therefore, claimall such in so far as they fall within the reasonable spirit and scopeof my invention.

Having .thus described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. In an electric induction fu-rnace for heating a travelling charge, asource of alternating current, an inductor coil supplied with currentfrom the source and a nonmagnetic metallic guide within the inductorcoil adapted to pass the charge.

k 2. man electric induction furnace for heating a travelling charge, asource of alternating current, an inductor coil supplied with currentfrom the source and non-magnetic metallic guide within the inductor andelectrically insulated from it, for guiding the charge travel throughthe inductor.

3. In an electric induction furnace for heating a travelling charge, asource of alternating current, an inductor coil supplied with currentfrom the source and a nonmagnetic metallic guide within the. inductorand electrically insulated.

from it.

4. In an electric induction furnace for heat-' -a spacing inductor ininductive relation with the primary inductor and also with a charge andwalls within the spacing inductor guiding the charge travel through it.

6. In an electric induction furnace for heating a travelling charge, asource of alternating with the primary inductor and also with a charge,

:wallsguiding the charge into the spacing ina travelling charge, asource of alternating cur-.

rent, an inductor supplied with current directly or inductively from thesource, non magnetic metallic guides within the inductor guiding travelof the charge through itand heat confining walls in prolongation of thepath through the inductor and wherein the charge is free from electricinduction.

8. In an electric induction furnace for heating a travelling charge, asource of alternating current, a primary inductor connected to thesource, a spacing inductor in inductive relation with the primaryinductor and also with a charge, walls within the spacing inductorguiding travel of the charge through it and heat confining walls inprolongation of the path through the spacing inductor. I

9. In an electric induction furnace, a source of alternating current, aprimary inductor connected to the source, a spacing inductor ininductive relation with the primary inductorand also with a continuousmetallic charge, walls within the spacing inductor guiding the travel ofthe charge and means operable at a point axially beyond the spacinginductor in the direction of travel of the charge for cutting off aportion 'of the charge periodically.

10. In an electric induction furnace, a source of alternating current, aprimary inductor connected to the source, a spacing inductor ininductive relation with the primary inductor and also with a charge,walls within the spacing inductor guiding travel of the charge throughit and a charge of nonoxidizinggas within th interior of the spacinginductor.

11. In an electric induction furnace a source of alternating current, aprimary inductor con nected to the source, a spacing inductor ininductive relation with the primary inductor and also with a charge andconsisting of a single turn from an electrical standpoint and wallswithin the spacing inductor guiding travel of the charge through thespacing inductor.

12. In an electric induction furnace. a source of alternating current, aprimary inductor consisting of a plurality of turns connected to thesource, a spacing inductor in inductive relation with the primaryinductor and also with acharge and comprising one turn from anelectrical standpoint and'walls Within the spacing inductor guidingtravel of the charge through the spacing of alternating current, aprimary inductor con-. nected to the source and a spacing inductorsurrounding the primary inductor comprising inductor bands electricallyin parallel and in the aggregate axially shorter than the primaryinductor. v v

15. man electric induction furnace, a source of "alternating current, aprimary inductor connected to the source and a focus inductorsurcurrent, a primary inductor connected to thesource, a spacinginductor in inductive relation rounding the primary inductor andcomprising a plurality of bands arranged side by side and united into asingle turn in length by strips extending-across the bands.

16. In an electric induction furnace, a. source of alternating current.a primary inductor connected to the source and comprising hollowfiattened edgewound tubing and a spacing inductor in inductive relationwith the primary inductor and also with the charge.

1'7. In an electric induction furnace, a source of alternating current,a primary inductor connected to the source and a spacing inductor ininductive relation with the primary inductor and also with the chargecomprising hollow flattened flatwound tubing.

18. In an electric induction furnace, a source of alternating current, aprimary inductor connected to the source and comprising hollow flattenededgewound tubing and a spacing inductor in inductive relation with theprimary inductor and also with the charge comprising hollow flattenedflatwound tubing.

19. In an 'electnc induction furnace, a source of alternating current, aprimary inductor connected to the source and comprising hollow fiattenededgewound tubing and a spacing inductor in inductive relation to theprimary inductor and also with the charge comprising hollow flattenedfiatwound tubing, a plurality being laterally connected to form a singleturn.

20. In an electric induction furnace, a source of alternating current, aprimary inductor connected to the source, a focus inductor in inductiverelation with the primary inductor and with the charge, having twoloops, one. of which is coaxial with and surrounding the primaryinductor and the other of which is bodily at one side of the primaryinductor, means for passing a metallic charge in the solid state throughthe side' focus inductor loop, and a shield enclosing the other focusinductor loop, whereby the oper- .tact with the stock within the heatingcoil and extending longitudinally of the coil and adapted to receive theweight of the stock.

v22. In an induction heater for travelling st ck,

a plurality of inductor turns in parallel, a p r ary and connections forinducing current in said turns as a secondary and a non-magneticmetallic guide and support for the travellingstock, extendingtransversely of the lower parts of the turns.

23. In an electric induction furnace for heat-. ing a travelling charge,a source of alternating current, an inductor coil supplied with currentI from the source and a non-magnetic metallic guide in contact with andsupporting the charge within the inductor coil whereby direct contactbetween the charge and the inductor coil is prevented and the charge issupported within the inductor coil, so that deformation of the portionof the charge within the induction coil due to its own weight isprevented.

EDWIN FITCH NORTHRUP.

